This is a letter written by Saichô (767-822, posthumously known as Dengyô Daishi), the founder of the Japanese Tendai sect of Buddhism. It is addressed to his most beloved disciple, Taihan, who at the time was studying at Takaosan-ji Temple (present-day Jingo-ji Temple) in Kyoto with Kûkai (774-835, posthumously known as Kôbô Daishi), the founder of the Shingon sect in Japan. This is the only extant letter in Saichô's own hand. It is called the "Kyûkaku Letter" (J. Kyûkakujô) because it begins with the phrase, "I have not heard from you in a long time" (J. kyûkaku seion).

On the twenty-third day of eleventh month of Kônin 4 (813), Saichô wrote a letter to Kûkai asking to borrow the texts Monju san hosshinrai ("Manjusri's Praises of the Dharma-body," also known as Ippyaku nijû raibutsu ["One Hundred Twenty Lines in Praise of the Buddha"]), Hôenzu ("Square and Round Charts"), and Chûgi ("Notes and Commentaries"), as well as Shaku rishukyô ("Interpretation of the Truth Sutra"). In response, however, Kûkai severely criticized Saichô for wanting to study esoteric Buddhism only through texts and refused to lend him Shaku rishukyô. Upon reading this, Saichô wrote Taihan the "Kyûkaku Letter" on the twenty-fifth day of the eleventh month, requesting that Taihan ask Kûkai in person the meaning of Monju san hosshinrai, Hôenzu, and Chûgi.

Previous to this, Saichô had received a poem from Kûkai, to which Saichô intended to compose a poem in response. However, the preface of Kûkai's poem mentioned the titles of the texts Monju san hosshinrai, Hôenzu, and Chûgi, all of which were unfamiliar to to the Tendai patriarch. Saichô, ever earnest in his dealings, wanted to read these texts himself in order to write an appropriate poetic response. Although he seems to have given up the idea of borrowing Shaku rishukyô, he at least wanted Taihan to ask about the contents of the unknown sources.

At this time, Saichô was forty-seven and Kûkai was forty. In the text of the letter, Saichô takes various measures to show courtesy to the younger Kûkai, as in always positioning Kûkai's title, Daiajari ("Great [Esoteric] Teacher"), at the beginning of a new line. This careful attention to etiquette reflects Saichô's sincere character. His handwriting also reveals a high level of sophistication.